1. Field of the Invention
The present invention relates to a multi-shaft electrically-operated automatic nut runner in which a plurality of driving shafts rotate by being electrically operated, and a plurality of transmission shafts are coupled with the driving shafts via torque-changing means, wherein an arrangement is provided such that a pitch circle diameter of sockets fixed to the transmission shafts is equal to a pitch circle diameter of wheel-mounting nuts of a vehicle, and the sockets are rotated to loosen or tighten the wheel-mounting nuts simultaneously.
2. Description of the Related Art
When the operation of removing and fitting a tire is performed for repairing a punctured tire of a vehicle, particularly a truck or a bus, for changing the tire with a new one, or for effecting the rotation of tires, the operator performs the operation by holding a pneumatic impact wrench with the vehicle jacked up. Since the impact wrench is heavy, the impact wrench is sometimes carried by being suspended from a crawler-type impact-wrench suspending machine.
Since an ordinary impact wrench has a single shaft, the operator consecutively loosens a plurality of (usually 8 to 10) wheel nuts by using the impact wrench to allow the tire to be removed.
However, the pneumatic impact wrench produces loud sounds and constitutes a source of noise. In addition, since the vibration is intense, the pneumatic impact wrench imposes a large burden (such as numbness of the hand) on the operator who holds it. Furthermore, there are cases where the pneumatic impact wrench undergoes fluctuations in the air pressure, which can cause variations in the value of tightening torque and, hence, variations in the tightening torque of each wheel nut.
To overcome the noises, vibrations, and torque fluctuations which are the drawbacks of the pneumatic impact wrench, it suffices to employ an electrically-operated automatic nut runner. With the electrically-operated automatic nut runner, however, since the number of revolutions of a driving source is reduced to obtain high torque, the operation time increases substantially, and the operating efficiency deteriorates.
To solve these problems, a multi-shaft automatic nut runner has been proposed which is provided with a multiplicity of shafts and is capable of simultaneously tightening a plurality of wheel nuts (refer to Japanese Utility-Model Application Publication No. 17787/1986, for example). The automatic nut runner disclosed in this publication has a variable pitch circle diameter and can therefore be made to correspond to pitch circle diameters of different vehicle types.
Consequently, the aforementioned automatic nut runner is capable of controlling the noises, vibrations, and torque fluctuations while maintaining an operating time equivalent to that of the pneumatic impact wrench.
The aforementioned automatic nut runner is used in the process of assembling vehicles. Namely, since the wheel nut enters the socket in a state in which the wheel nut is loosened with respect to an inner nut, the wheel nut can be rotated (tightened) with a relatively low torque at the beginning of fitting in. Even if the sockets are fitted over the respective wheel nuts with time lags, the operation is not affected much.
However, in the process of loosening the wheel nuts, i.e., in a process such as the operation of changing the tire, since a maximum torque is required from the outset when the sockets are fitted over the respective wheel nuts, if even one socket is fitted over the wheel nut, a force for rotating the entire automatic runner is generated centering around that fitted shaft, so that the other sockets fail to be fitted over the other wheel nuts.